This invention relates to a planar high breakdown voltage semiconductor device.
Planar semiconductor devices are characterized in that the reliability is high and they are suitable for manufacturing in the basis of mass production but they have been previously disadvantageous in that high breakdown voltage elements are difficult to produce. One of the reasons for this is the concentration of an electric field involved on the curved portion of the PN junction. Furthermore in planar semiconductor devices including the silicon substrate, a positive surface charge is present at the interface between the silicon substrate and a film of silicon dioxide disposed thereon and affects the substrate so that the depletion layer therein is difficult to spread on the surface of the silicon substrate resulting in the facilitation of the concentration of the electric field on the surface of the substrate. This is another cause for which planar high breakdown voltage elements are difficult to be produced.